The research involves five projects. The first is a summary and functional analysis of the afferent connectivity of somatic sensory and adjacent motor portions of the cat's mesencephalon and diencephalon. The second is a similar analysis in the monkey. Both projects involve the light microscopic (LM) examinations using a differential orthograde labeling strategy in which autoradiographic and degeneration tracing methods are combined in the same preparation. The third project involves a variety of LM experiments designed to examine the mechanisms which produce differential incorporation patterns of 3H-leucine and 3H-proline within the dorsal column nuclei (DCN) of cats. Some of the manipulations which are being made involve changes in the ionic characteristics of the injection vehicle, the survival time, the functional status of the cells in DCN, and the fixative, as well as comparisons with other precursor molecules. A possible glial participation in the transport of 3H-proline from DCN to the inferior olive (IO) is being examined using the retrograde transport of horseradish peroxidase (HRP) from IO to DCN as visualized at both the light and electron microscopic levels. The fourth project is designed to study the relationship between the patterns of afferent input to portions of cat and monkey thalamus and the efferent projections from these portions to the cerebral cortex. This project uses a triple labeling strategy in which the retrograde transport of HRP is added to the differential orthograde procedures. The fifth project is a series of LM experiments using two retrogradely transported labels (HRP and 3H-apoHRP) to study the possible collateralization of the efferent projections of DCN neurons, olivocerebellar neurons and motoneurons.